1. Field of the Invention
This invention relates to an exhaust gas purification system of an internal combustion engine, particularly to an exhaust gas purification system of an internal combustion engine that uses a selective-reduction type NOx reduction catalyst to reduce NOx (oxides of nitrogen) constituents in an exhaust gas in an oxidizing state by use of HCs (hydrocarbons) as a reducing agent.
2. Description of the Related Art
U.S. Pat. No. 5,357,749, for example, teaches a system for purifying exhaust gas by using a catalyst that decomposes NOx (oxides of nitrogen) constituents in oxidizing environment (lean air/fuel ratio), particularly a selective-reduction type NOx reduction catalyst that reduces NOx constituents in an oxidizing environment, and regulating the exhaust constituent concentrations such that the NOx constituent concentration and HC constituent concentration in the exhaust gas are regulated to a prescribed ratio.
This conventional system utilizes the fact that the NOx reduction (decomposition) catalyst optimally purifies both NOx and HC at a particular NOx--HC ratio, namely, at a particular ratio of the NOx and HC concentrations. Specifically, the system purifies NOx in an oxidizing atmosphere by detecting the NOx and HC concentrations of the exhaust and controlling the detected concentrations to prescribed values by regulating the air/fuel ratio, amount of secondary air, ignition timing and/or other parameters.
One of the known decomposition catalysts of this type is the selective-reduction type NOx reduction catalyst taught by, for example, U.S. Pat. No. 5,326,735 and U.S. Pat. No. 5,487,268, which comprises iridium and an alkaline earth metal carried together on a substrate (monolith) comprised of at least one material or substance selected from the group comprised of metallic carbides and metallic nitrides. Another is the catalyst taught by U.S. Pat. No. 5,402,641, which is an NOx-absorbent comprising platinum (Pt) or like noble metal borne on a substrate (monolith) and referred as an absorbent catalyst.
Recently, however, the move toward leaner air/fuel ratio controls, as seen in lean-burn engines and direct injection engines (in which fuel is directly injected in the engine cylinders), has created a need for higher NOx constituent purification performance in an oxidizing environment (i.e., under leaner air/fuel ratio).